What three documents need to be checked and prepared before an invoice can be processed for payment?

First, it would be very nice if you would give us your attempt to answer the question rather than expecting to just get an answer. You need to think about it for yourself first.

Second, sometimes these things are book-dependent. I've worked in strictly payables twice and can only think of two documents I'd feel it necessary to check. If they want three, either it's something unusual, or they're taking what I'd call one document and splitting it somehow, or they're including the invoice itself? (To me, checking the invoice is part of "preparing and processing" it. This is why it's not fun when an unknown book says there's x number of something or x number of ways to do something.)

The original purchase order, the invoice and the final demand letter requesting payment. Sorry but I couldn't help the last one.